LANDSLIDES AND STREAM CAPTURE IN THE SOUTHERN MAYACAMAS MOUNTAINS, CALIFORNIA

Geomorphic mapping has shown that a significant percentage of the slope area in the southern Mayacamas Mountains of northern California consists of landslide terrain. Some of these slope failures have caused large-scale changes to hillside topography. Displacement of stream channels by large landslides is common throughout the study area. In some cases, displacement of stream channels by large and deep-seated landslides has led to stream capture. In the Kellogg Creek watershed, an example of stream capture due to landslide displacement is explored. In this example, a south-flowing tributary to Kellogg Creek was displaced from its original channel, leaving a beheaded drainage disconnected from any watershed. The beheaded drainage is truncated by the lateral margin of a large, deep-seated landslide that failed into the stream channel, creating a high scarp that truncates the beheaded channel. The active stream channel now turns eastward where it was displaced by the large landslide and descends into a steep-sided canyon that was cut by south-flowing Kellogg Creek. Deep-seated landsliding in the Mayacamas Mountains has resulted from a variety of factors, including high average rainfall, weak volcanic and sedimentary rocks, steep mountain slopes, and strong seismic shaking. These factors have resulted in development of drainage networks that have been extensively modified by large landslides.